Electrolytic reductive coupling process



United States Patent M 3,274,084 7 ELECTROLYTIC REDUCTIVE CUUPLWG PROCESS Manuel M. Baizer, St. Louis, Mo., assignor to Monsanto Company, St. Louis, Mo., a corporation of Delaware No Drawing. Filed July 1, 1965, Ser. No. 468,997 8 Claims. (Cl. 20 1-72) This application is a continuation-in-part of my application S.N. 216,306, filed August 13, 1962, and issued as Patent No. 3,193,479 on July 6, 1965; and a continuation-in-part of my application S.N. 189,072, filed April 20, 1962, and issued as Patent No. 3,193,477 on July 6, 1965.

This invention relates to the manufacture of polyfunctional compounds and more particularly provides a new and valuable electrolytic process for reductive coupling, i.e. hydrodirnerization, of u,;3-olefinic ketones in aqueous quaternary ammonium salt electrolyte solution.

It is an object of the invention to provide a process for the preparation of di-functional compounds, particularly diketo compounds, which are useful as intermediates in various organic reactions.

According to the presently provided process, reduced coupled products of olefinic ketones are produced as follows:

in which each R is individually selected from the group consisting of hydrogen, alkyl (including cycloalkyl) and aryl radicals, particularly such radicals containing up to 8 carbon atoms. It will be recognized that a wide variance in the substituents is permissible, and each individual R can be the same as or different from another R, and that the foregoing examples are illustrative rather than limiting, as a, 8-o1efinic ketones in general are suitable for use in the present process. While the illustrative formula shows only one functional group, it will be recognized that each olefinic reactant can have two or more such functional groups.

In general, the electrolytic reductive coupling is conducted in concentrated solution in aqueous quaternary ammonium salt electrolyte. Quaternary ammonium sulfonates and alkyl sulfates are particularly suitable as the supporting electrolyte salt. It is desirable to employ fair- 1y concentrated solutions in order to minimize undesired reactions of intermediate ions with the water of the electrolyte. The olefinic reactants will generally comprise at least 10% by weight of the electrolyte, and preferably at least 20% by weight or more. It is generally desirable to employ fairly high concentrations of salts in the electrolyte, for example constituting and usually 30% or more by weight of the total amount of salt and water in the electrolyte, in order to obtain the desired solubility of the olefinic compounds. To some extent it is possible to obtain the effect of high olefin concentration by lowering the water concentration, for example by employing a co-solvent and having the water constitute less than 5% by weight of the electrolyte salt and co-solvent.

Some u,fi-olefinic ketones are subject to side reactions if the electrolyte is acidic, and it is desirable in such cases to maintain non-acidic conditions during the electrolysis, although many of the quaternary ammonium salts employed are naturally slightly alkaline or at least not greatly acidic in nature, so no special provisions with regard to pH are needed. However, the pH during the present 3,274,084 Patented Sept. 20, 1966 process is generally in the range of about 6 to 12, preferably about 7 to 9.5.

In eifecting the reductive coupling of the present invention it is preferred to utilize a cathode having an overvoltage greater than that of copper and to subject to electrolysis in contact with such cathode a concentrated solution of a mixture of the defined olefinic compounds in an aqueous electrolyte under mildly alkaline conditions. In effecting the reductive couplings of the present invention, it is essential to obtain cathode potentials required for such couplings and therefore the salt employed should not contain cations which are discharged at numerically lower, i.e., less negative, cathode potentials, but rather the electrolyte should have a half-wave potential substantially more negative than that at which the desired coupling occurs. It is desirable that the salt employed have a high degree of water solubility to permit use of very concentrated solutions for concentrated salt solutions dissolve greater amounts of the organic olefinic compounds. The quaternary ammonium salt solutions employed in the present invention have highly negative discharge potentials, and are highly soluble when the anions are sulfonates or alkyl sulfates.

The coupled product generally obtained in the present invention is that resulting from coupling at the carbon atom of the olefinic bond in the beta position with respect to the keto group.

It will be recognized that the required cathode potentials for hydrodimerization will vary somewhat with the conditions employed, but benzalacetone can be hydrodimerized at -1.3 volts, mesityl oxide at -1.58 to 1.73 volts, and methyl vinyl ketone at -1.43 volts, the potentials being those measured against a saturated calomel electrode. Examples of other unsaturated ketones useful in the present process are Z-cyclohexenone, isopropylidenacetone, benzalacetophenone, 4-methoxybenzalacetophenone, dibenzalacetone, ethylideneacetophenone, methylionione, heptadiene-3,5-one-2, carvone, etc., the required cathode potentials approximating the half-wave potentials of the compounds, eg, -1.55 volts for 2- cyclohexenone and -1.61 volts for isopropylidenacetone. The electrolysis is carried out so as to cause development of the cathode potentials necessary for hydrodimerization. For the most part the olefinic ketones employed .will be mono-olefinic ketones which are hydrocarbon except for the keto groups and which contain from 4 to about 12 carbon atoms.

The following examples illustrate the invention.

Example 1 A catholyte was prepared by dissolving 81.5 grams benzalacetone in an aqueous solution of 20.5 grams water and 82.5 grams tetraethylammonium p-toluenesulfonate, along with a trace of hydroquinone as stabilizer. The catholyte was placed in a jacketed glass vessel containing ml. mercury as cathode. A platinum anode was employed in contact with an aqueous solution of the sulfonate salt as anolyte, which was placed in an Alundum cup which was partially immersed in the catholyte. Electrolysis was effected at cathode potentials of -1.26 to -1.31 volts (vs. saturated calomel electrode) at a 2.3 ampere current for a total of 5.1 ampere-hours. Intermittent addition of glacial acetic acid to a total of 1.55 ml. was employed to control alkalinity. The temperature during the electrolysis rose to 50-60 C. The electrolysis medium was extracted with 50 ml. portions methylene chloride and the extracts washed with water and then distilled to give 20.8 grams of crude 4,5-diphenyl-2,7- octanedione, RP. 188190 C. (0.17-0.21 min). Crystallization from alcohol yielded white crystals of the compound, M.P. 163 C. (Polarography, Kolthoit and Lingane, Interscience Pub., New York, N.Y., 1952, vol. 2,

. chapt. 38, reports 161 C.).

A 27.5 gram amount of the benzalacetone was recovered. No benzylacetone was found by vapor phase chromatography. The yield based on current was about 83%.

Example 2 A cartholyte was prepared from 70 grams mesityl oxide, 56.2 grams tetraethylammonium p-toluenesulfonate, 14 grams water and 22.3 grams acetonitrile. The catholyte was electrolyzed in contact with a mercury cathode at -1.59 to 1.79 cathode volts (vs. saturated calomel electrode) in a cell as described in Example 1. The electrolysis at 2.3 amperes was continued for 9.3 amperehours. The product was isolated by extraction and distillation to give the 4,5-tetramethyl-2,7-dione hydrodimers along with some re-arrangement products, 12.4 grams, B. P. 98110 C. (20 mm.), 11 1.4669; 3.2 grams, B.P. 110122 C. (20 mm.), 11 1.4655; 12.8 grams, B.P. l23132 C. (20 mm.), 11 1.4660. The yield of the foregoing was 83%. The hydrodimer is useful for forming a cyclic compound by known procedures, the compound being 1-aceto-2-hydroxy-2-methyl-4,5,-tetramethylcyclopentane.

During electrolysis in a divided cell, alkalinity increases in the catholyte. However, the anolyte becomes acidic. When a porous diaphragm is used to separate the catholyte from the anolyte, the alkalinity of the catholyte will depend upon the rate of diffusion of acid from the anolyte through the porous barrier. Control of alkalinity in the catholyte, when employing a diaphragm, may thus be realized .by purposely leaking acid from the anolyte into the catholyte. It can also be achieved, of course, by extraneous addition to the catholyte of an acid material, e.g. glacial acetic acid, phosphoric acid or p-toluenesulfonic acid. Alkalinity may also be controlled, whether or not a diaphragm is used in the cell, by employing buffer systems of cations which will maintain the pH range while not reacting at the reaction conditions.

When a divided cell is employed, it will often be desirable to use an acid as the anolyte, any acid being suitable particularly dilute mineral acids such as sulfuric or phosphoric acid. Hydrochloric acid can be employed but would have the disadvantage of generating chlorine at v the anode, and of being more corrosive with respect to some anode materials. When an acid is employed as anolyte, it is advantageous to use an ion exchange membrane to separate the anolyte from the catholyte. If desired, a salt solution can be used as anolyte, those useful as catholyte also being suitable as anolyte, although there are many other salt solutions suitable for such use.

Materials suitable for constructing the electrolysis cell employed in the present process are well known to those skilled in the art. The electrodes can be of any suitable cathode and anode material. The anode may be of virtually any conductor, although it will usually be advantageous to employ those that are relatively inert or attacked or corroded only lowly by the electrolytes; suitable anodes are, for example, platinum, carbon, gold, nickel, nickel silicide, Duriron, lead and lead-antimony and lead-copper alloys, and alloys of various of the foregoing and other metals.

Any suitable material can be employed as cathode, various metals and alloys being known to the art. It is generally advantageous to employ metals of fairly high hydrogen overvoltage in order to promote current elficiency and minimize generation of hydrogen during the electrolysis. In general it will be desirable to employ cathodes having overvoltages at least about as great as that of copper, as determined in a 2 N sulfuric acid solution at current density of 1 milliamp/square centimeter (Carman, Chemical Constitution and Properties of Engineering Materials, Edward Arnold and Co., London, 1949, page 290). Suitable electrode materials include, for example, mercury, cadmium, tin, zinc, bismuth, lead,

graphite, alumimun, nickel, etc., in general those of higher overvoltage being preferred. It will be realized that overvoltage can vary with the type of surface and prior history of the metal as well as with other factors; therefore the term overvoltage as used herein with respect to copper as a gauge has reference to the overvoltage under the conditions of use in electrolysis.

As the salts which can be employed in the present invention the quaternary ammonium salts are generally suitable, especially those of sulfonic and alkyl sulfuric acids, Those suitable include aliphatic and heterocyclic quaternary ammonium salts, i.e., the tetraalkylammonium or the tetraalkanolammonium salts or mixed alkyl alkanol ammonium salts such as the alkyltrialkanolammonium, the suitably high cathode discharge potentials for use in the or the N-heterocyclic N-alkyl ammonium salts of sulfonic or other suitable acids. The saturated aliphatic or heterocyclic quartenary ammonium cations in general have slightly high cathode discharge potentials for use in the present invention and readily form salts having suitably high water solubility with anions suitable for use in the electrolytes employed in the present invention. The saturated, aliphatic or heterocyclic quaternary ammonium salts are therefore in general well adapted to dissolving high amounts of olefinic compounds in their aqueous solutions and to effecting reductive couplings of such olefinic compounds. It is understood, of course, that it is undesirable that the ammonium groups contain any reactive groups which might interfere to some extent with the reductive coupling reaction. In this connection it should be noted that aromatic unsaturation as such does not interfere as benzyl substituted ammonium cations can be employed (as also can aryl sulfonate anions).

Among the anions useful in the electrolytes, the aryl and *alkaryl sulfonic acids are especially suitable, for example, salts of the following acids: benzenesulfonic acid, 0-, mor p-toluenesul-fonic acid, o-, mor p-ethylbenzenesulfonic acid, o-, mor p-cumenesulfonic acid, o-, mor p-tert-amylbenzenesulfonic acid, o-, mor p-hexylbenzenesulfonic acid, o-xylene-4-sulfonic acid, p-xylene-2- sulfonic acid, m-xylene-4 or 5-sulfonic acid, mesitylene- 2-sulfonic acid, durene-3-sulfonic acid, pentamethylbenzenesulfonic acid, 0-dipropylbenzene-4-sulfonic acid, alphaor beta-naphthalenesulfonic acid, o-, mor p-biphenylsulfonic, and alpha-methyl-beta-naphthalenesul- 5 fonic acid.

Presently useful quaternary ammonium sulfonates are, e.g.,

tetraethylammonium o-, or m-toluenesulfonate or benzenesulfonate; tetraethylammonium o-, mor p-cumenesulfonate or o-, mor p-ethylbenzenesulfonate, tetramethyl ammonium benzenesulfonate, or o-, mor p-toluenesulfonate; N, N-di-methylpiperidinium, o-, mor p-t-oluenesulfonate or o-, mor p-biphenylsulfonate; tetrabutylammonium alphaor beta-naphthalenesulfonate or o-, mor p-toluenesulfonate; tetrapropylammonium 0, mor p-amylbenzenesulfonate or alpha-ethyl-beta-naphthalene s-ulfonate; tetraethanolammonium o-, mor p-cumenesulfonate or o-, mor p-toluenesulfonate; tetrabutanolammonium benzenesulfonate or p-xylene- 3-sulfonate; tetrapentylanunonium o-, mor p-toluenesulfonate or o-, mor p-hexylbenzenesulfonate, tetrapent-anolammonium p-cymene-3-sulfonate or benzenesulfonate; methyltriethylammonium -o-, mor p-toluenesulfonate or mesitylene-Z-sulfonate; trimethylethylammonium o-xylene-4-sul'fonate or o-, mor p-toluenesulfonate; triethylpentylammonium alphaor beta-naphthalenesulfonate or o-, mor p-butylbenzenesulfonate, trimethylethanolammonium benzenesulfonate or o-, mor p-toluenesulfonate;

N,N-di-ethylpiperidinium or N-methyl-pyrrolidinium 0-, mor p-hexylbenzenesulfonate or o-, mor p-toluenesulfonate, N,N-diisopropyl or N,N-

di-butylmorpholinium o-, mor p-toluenesulfonate or 0-, mor p-biphenylsulfonate, etc.

Among the ammonium sulfonates useful as electrolytes in the present invention are the alkyl, aralkyl and heterocyclic ammonium sulfonates, in which ordinarily the individual substituents on the nitrogen atom contain no more than atoms, and usually the ammonium radical contains from 3 to carbon atoms. It will be understood, of course, that diand poly-ammonium radicals are operable and included by the term ammonium. The sulfonate radical can he from aryl, alkyl, alkaryl or aralkyl sulfonic acids of various molecular Weights up to for example 20 carbon atoms, preferably about 6 to 20 carbon atoms, and can include one, two or more sul- -fonate groups.

Another especially suitable class of salts for use in the present invention are the quaternary ammonium alkylsulfate salts such as ethylsulfate and methosulfate salts, and such salts of other alkyl sulfates containing 1 to 8 carbon atoms in the alkyl group. Methosulfate salts such as the methyltriethylammonium, tri-n-propylmethylammonium, ttriamylmethylammonium, tri-n-butylmethylammonium, etc., are very hygroscopic, and the tri-n-butylmethylammonium in particular forms very concentrated aqueous solutions which dissolve large amounts of organic materials, and tetraethylammonium ethylsulfate is also very suitable. In general ammonium cations suitable for use in the alkylsulfate salts are the same as those for the sulfonates.

In general the quaternary ammonium salts employed as electrolyte will be those classed as hydrotropic or improving the solubility of organic compounds in water The diketo compounds produced by the present process are in general known compounds of known properties, such as the capability of rearrangement to cyclic compounds by heating in the presence of catalyst.

What is claimed is:

1. The method of producing a reduced, coupled product which comprises subjecting an aqueous solution of 8- olefinic ketone in contact with a cathode to electrolysis, the said aqueous solution containing a quaternary ammonium salt, and recovering a reduced, coupled product of said ketone from the aqueous solution.

2. The method of claim 1 in which the salt is a quarternary ammonium aromatic sulfonate,

3. The method of claim 1 in which the salt is a quarternary ammonium alkylsulfate.

4. The method of hydrodimerizing an a,fi-olefinic ketone which comprises subjecting a solution of 0:,[3-016- finic ketone to electrolysis in contact with a cathode having an overvoltage greater than that of copper, causing development of the cathode potential required for hydrodi merization of the ketone, the said solution containing at least 10% by weight of the ketone and at least 5% by weight of quarternary ammonium salt and having a pH of at least 7 during the electrolysis, and recovering hydrodimer of said ketone from said solution.

5. The method of claim 4 in which mesityl oxide is hydrodimerized.

6. The method of claim 4 in which benzalacetone is hydrodimerized.

7. The method of claim 4 in which the salt is a tetraalkylammonium arylsulfonate containing up to 20 carbon atoms in the ammonium group and from 6 to 20 carbon atoms on the sulfonate anion.

8. The method of claim 4 in which the salt is a tetraalkylammonium alkylsulfate containing up to 20 carbon atoms in the ammonium group and up to 8 carbon atoms in the alkyl sulfate anion.

No references cited.

JOHN H. MACK, Primary Examiner.

H. M. FLOURNOY, Assistant Examiner. 

1. THE METHOD OF PRODUCING A REDUCED, COUPLED PRODUCT WICH COMPRISES SUBJECTING AN AQUEOUS SOLUTION OF A,BOLEFINIC KETONE IN CONTACT WITH A CATHODE TO ELECTROLYSIS, THE SAID AQUEOUS SOLUTION CONTAINING A QUATERNARY AMMONIUM SALT, AND RECOVERING A REDUCED, COUPLED PRODUCT OF SAID KETONE FROM THE AQUEOUS SOLUTION. 